A liquid crystal display is being used as the most important display device in the multimedia society over a wide range, including a cellular phone, a computer monitor, a laptop computer and a television set. In a liquid crystal display, many optical films are used so as to enhance the display characteristics, and among others, a retardation film fulfills a great role in, for example, improving the contrast or compensating the color tone when the display is viewed from the front or oblique direction. As the conventional retardation film, a polycarbonate or a cyclic polyolefin is used, and all of these polymers are a polymer having positive birefringence.
The polymer having negative birefringence includes an acrylic resin and a polystyrene, but the acrylic resin is a small retardation and is insufficient in the characteristics as a retardation film. The polystyrene is not used at present, because this polymer has a problem with retardation stability, such as occurrence of a change in the retardation with a slight stress due to its large photoelastic coefficient in a low temperature region; a problem in view of optical characteristics, such as large wavelength dependency of the retardation; and a problem in practical use, such as low heat resistance.
To meet the market requirement for a retardation film having negative birefringence, a fumaric acid diester-based resin and a film composed of the resin have been proposed (see, for example, Patent Documents 1 to 5).